1. Field of the Invention
The present invention relates in general to permanent magnet structures and more particularly to magnet structures providing a guidance mechanism in a Free Electron Laser for continuous high energy and high frequency radiation.
2. Description of the Prior Art
Magnetic fields are commonly used in the operation of numerous electronic devices, including those that accelerate and focus electrons in a free electron laser. A free electron laser typically has a passageway, which is often an excavated cylindrical space, through which an electron beam can pass and made to oscillate in a direction perpendicular to its translation velocity. The bar magnet array produces a field which alternates in the directions shown by the large arrows in FIG. 1, because the magnetization of the individual magnets reverses its direction from magnet to magnet, as indicated by the small arrows in FIG. 1. This causes an electron beam passing axially along the array to wiggle in a direction perpendicular to both the translational, or axial, velocity and the field direction in accordance with elementary electromagnetic theory. This wiggle constitutes an acceleration and an acceleration of an electric charge causes radiation, which in this case is propagated substantially in the axial direction and is polarized in a mutual plane of the wiggle motion and the axial velocity shown in FIG. 1.
In such an arrangement, the electron beam makes only one pass through the magnetic xe2x80x9cwigglexe2x80x9d array and much of the electron beam energy is typically wasted. Recently, other techniques for circular wigglers have been produced in which the electron beam can make multiple passes through the periodic magnet array. These prior art concepts are disclosed in U.S. Pat. No. 5,723,949, 5,719,469, 5,705,064 and 5,666,098. These prior art concepts are also exemplified in these publications: Herbert A. Leupold, Ernest Potenziani II. and Anu S. Tilak, xe2x80x9cCircular Free-Electron Laser Structures,xe2x80x9d Journal of Applied Physics, Volume 81, No. 8, p. 5144 (April 1997); and also Herbert A. Leupold, Anu S. Tilak and Ernest Potenziani II, xe2x80x9cToroidal Electron Beam Radiation Sources,xe2x80x9d Transactions IEEE Magnetics 33, No. 5, p. 3418 (1997).
All of these circular free electron lasers emit synchrotron, as well as, wiggler radiation, in which the frequency of the wiggler radiation is N times the frequency of the synchrotron radiation, where N is the number of magnet periods in a circuit. The two types of radiation are emitted tangentially from the circular beam and are polarized in the plane of the electron beam circulation. It is often desirable to have a monochromatic radiation beam and therefore eliminate the synchrotron component of radiation. Separation of the two components is easier if the polarizations are normal to each other, in which case, one component or the other can be eliminated by various polarization devices. It is the purpose of the present invention to produce a circular-beam free electron laser in which the emitted synchrotron and wiggler radiations are normal to each other.
In the prior art magnetic structures, the circulation of the electron beam itself produces synchrotron radiation, i.e. radiation emanating from a centripetal acceleration wherein the charged particle""s velocity changes direction. Referring now to the FIGS. 2A and 2B prior art toroid, FIG. 2A depicts an expanded view of toroid 10 with its iron core slotted to form an azimuthal array of iron teeth 11. The structure""s electron path is indicated by arrow 12, the magnetic flux path is indicated by arrow 13 and small arrows 14 depict the magnetization of the permanent magnet toroidal shell.
The iron teeth 11 in toroid structure 10 cause a radial oscillation of the electron beam because of the periodicity of the field strength that is engendered by the periodicity of the iron, namely the sequence of alternate teeth 11 and gaps. Arrows 15 show the direction of the acceleration of the electron beam in the radial direction. Magnetic field 13 keeps the electron beam within its circular path. When an electron is between teeth 11, it is accelerated inwardly because there the field is stronger than necessary to keep the electron beam on a circular path. However, when the electron is between the gaps that separate teeth 11, the electron beam is accelerated outwardly because at that point the field is insufficiently strong to keep to that path. This results in an oscillation around the average circular trajectory. The wiggler radiation results from this oscillation which constitutes an acceleration. The synchrotron radiation results from centripetal acceleration necessary to keep the beam on a circular path. The frequency of the resulting wiggler radiation is then equal to the number of teeth, N, times the frequency of the synchrotron radiation because N tooth-gap pairs constitute N wiggles of the electron per circuit. FIG. 2B depicts an external view of toroid structure 10.
The wiggler and synchrotron radiation are both emitted tangentially from the toroidal azimuthal axis and they are both polarized in the toroid""s principal plane, as described above. The present invention polarizes the two different radiation sources normal to each other. By polarizing the two different radiation sources normal to each other in a mutually orthogonal arrangement, then either one radiation or the other could be easily removed by a polarizing element such as an appropriate prism. Such an arrangement would permit the same magnetic field source to confine an electron beam to a circular path and provide lateral periodic motion for wiggler rotation. The present invention""s polarization of two different radiation sources normal to each other in a mutual orthogonal arrangement provides monochromatic radiation sources for radars, communications, electronic warfare, medical diagnostics and so on
References on toroidal and spherical magnetic structures include U.S. Pat. No. 5,486,802 xe2x80x9cSpherical Magnet Structure And Use thereof In Synchrotron Radiation Source,xe2x80x9d and U.S. Pat. No. 5,716,469 xe2x80x9cSpherical Magnet Having a Gap With a Periodically Varying Field For a Wiggler Radiation Source,xe2x80x9d which were issued to the inventor herein on Jan. 23, 1996 and Feb. 17, 1998, respectively.
It is an object of the present invention to provide a permanent magnet structure to polarize the wiggler and synchrotron radiation normal to each other.
It is an additional object of the present invention is to provide a permanent magnet structure to polarize the wiggler and synchrotron radiation sources normal to each other with a series of wedge-shaped magic ring sections forming a toroid having an equatorial gap extending about the periphery of the toroid.
These and other objects are advantageously accomplished with the present invention by constructing a toroid from a series of wedge-shaped magic ring sections in a way that polarizes the two wiggler and synchrotron radiation sources normal to each other in a mutually orthogonal fashion. This arrangement allows either one radiation or the other to be easily removed by a polarizing means such as an appropriate prism. Further, in accordance with the present invention, the magnetic field source can confine an electron beam to a circular path and provide lateral periodic motion for wiggler radiation.